For a garden then theres no reason you cannot use Gimp. Just set the actual real world scale to pixels ratio like 1cm per pixel or whatever suits the size of garden your designing. I designed bits of my house using my own fantasy mapping app and it worked out fine and I would use it again for similar projects.

The main advantages of some GIS apps is that they could calculate areas or curvy lengths or a number of other things as a function within the app. With Gimp you would need to read off the line coords and convert them to real world all the time. But if you use some sensible pixel to metres scale then its no bother.

The advantage of Gimp over GIS / CAD tools is that you can easily get images of trees and some of the plants that you may want to use.